(1) Field of the Invention
The present invention relates to telemetry for sampling individual receiving modules of a towed array and more particularly to a system which utilizes only two coaxial cables and a plurality of identical receiving modules to construct a low cost, expendable, small diameter towed array.
(2) Description of the Prior Art
Presently used towed acoustic receiving arrays are very costly; normally use a complex telemetry system with a unique data acquisition telemetry module for each channel of data; are relatively large in diameter (2-3 inch) which causes storage, deployment and retrieval problems; typically use flammable fill fluid to achieve neutral buoyancy and to help dissipate internal heat; and usually require relatively high electrical power (1 watt or more) per array acoustic channel. Recovery of an array's many separate channels of acoustic data is necessary to improve array sensitivity and directivity in conjunction with modern array signal processing.
Multichannel telemetry data can be coded in many ways depending on the data bandwidth, the signal dynamic range, the number of channels required, and the telemetry cable limitations. Digital, analog and hybrid telemetry methods are presently used. A digital time division multiplexed format allows telemetry of many channels with wide dynamic range to be coupled as necessary over a distance of many thousands of meters using digital repeaters. However, the data cable must be larger in diameter to allow passage of the relatively wide telemetry signal bandwidth. In addition, power per channel tends to be high due to high speed clocking requirements. To maintain channel identity, a different address number is normally assigned to each channel. Unless a separate plug-in address module is used, this requirement limits interchangeability.
To convey acoustic information, analog multichannel carrier techniques use either frequency modulation (FM) or amplitude modulation (AM) of high frequency carriers. A single cable can pass up to approximately 20 FM channels before carrier drift and crosstalk become a problem. AM-FM techniques can carry more data by bandshifting groups of modulated carrier data, but circuit complexity and power increase and extremely linear repeaters are usually required to prevent harmonic distortion interference. Amplitude modulation allows more channels to exist in a given limited cable bandwidth, especially if one of several single sideband methods are used; however, the received signal level varies inversely with the cable length and low frequency response is usually poor due to the modulation limitations. Both FM and AM methods require that each data acquisition module must operate at a unique carrier frequency; therefore, particular module sparing and repairing problems arise.